Draft: [RFC] temp/alpine-appstream-downloader: new aport

So after going again through #1384 (closed) and working for longer in GS, I came to the conclusion that the best approach to have maintainable, long-term working AppStream data in pmOS and alpine, for both PD and GS is to have a small tool which makes sure that we are compliant with the specification. I wrote and tested such a small python script. It takes care of downloading the data from https://appstream.alpinelinux.org matching against /etc/apk/repositories, to make sure that we are always downloading the correct data.

I tried to make the program self-managed, so that it would run as a daemon and fetch data when /lib/apk/db/lock was locked and unlocked. But I wasn't able to make it work with the exclusive lock required by apk. So to fully integrate this, it would need to be called from PD polkit process (ideally I guess that when pulling updates), and from apk-polkit-rs. Hopefully calling such a simple script should be something trivial to implement, and would avoid the extra patch @minlexx is maintaining for downloading data, and making use of buggy and non-compliant external-appstream-urls from GS.

An additional benefit from having a small script is that it could be called from pmbootstrap during the process of building images, so that phosh/gnome and plasma images come with the appstream data already installed. This should make a much more pleasant experience on the first use of GS, and I guess also PD, but that I cannot tell. For the record, in my edge installation, the take this amount of space:

user@alpine-edge:~/data/Informatica/postmarketOS/pmaports$ du -hd1 /var/cache/swcatalog/icons/
3.3M    /var/cache/swcatalog/icons/alpinelinux-edge-community
1.5M    /var/cache/swcatalog/icons/alpinelinux-edge-testing
32K     /var/cache/swcatalog/icons/alpinelinux-edge-main
4.7M    /var/cache/swcatalog/icons/
user@alpine-edge:~/data/Informatica/postmarketOS/pmaports$ ls -lh /var/lib/swcatalog/xml/
total 3.8M
-rw-r--r-- 1 root root 3.5M Dec 22 23:39 alpinelinux-edge-community.xml.gz
-rw-r--r-- 1 root root 9.8K Dec 22 23:38 alpinelinux-edge-main.xml.gz
-rw-r--r-- 1 root root 287K Dec 22 23:39 alpinelinux-edge-testing.xml.gz

So please, thoughts 😄

Relates #1384 (closed)

Ah, and I forgot. Of course this shouldn't be merge here, but in alpine. Although pmOS are the main receivers of this effort, so thus the discussion here.